Numerical Investigation on Fracture Initiation Properties of Interface Crack in Dissimilar Steel Welded Joints

Fracture toughness property is of significant importance when evaluating structural safety.The current research of fracture toughness mainly focused on crack in homogeneous material and experimental results.When the crack is located in a welded joint with high-gradient microstructure and mechanical property distribution,it becomes difficult to evaluate the fracture toughness behavior since the stress distribution may be affected by various factors.In recent years,numerical method has become an ideal approach to reveal the essence and mechanism of fracture toughness behavior.This study focuses on the crack initiation behavior and driving force at different interfaces in dissimilar steel welded joints.The stress and strain fields around the crack tip lying at the interfaces of ductile-ductile,ductile-brittle and brittle-brittle materials are analyzed by the numerical simulation.For the interface of ductile-ductile materials,the strain concentration on the softer material side is responsible for ductile fracture initiation.For the ductile-brittle interface,the shielding effect of the ductile material plays an important role in decreasing the fracture driving force on the brittle material side.In the case of brittle-brittle interface,a careful matching is required,because the strength mismatch decreases the fracture driving force in one side,whereas the driving force in another side is increased.The results are deemed to offer support for the safety assessment of welded structures.

Interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints during aging

The interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints with a nickel-based filler metal during aging was studied in detail to elucidate the mechanism of premature failures of this kind of joints.The results showed that not only a band of granular Cr_(23)C_(6)carbides were formed along the fusion boundary in the ferritic steel during aging,but also a large number of granular or plate-like Cr_(23)C_(6)carbides,which have a cube-cube orientation relationship with the matrix,were also precipitated on the weld metal side of the fu-sion boundary,making this zone be etched more easily than the other zone and become a dark etched band.Stacking faults were found in some Cr_(23)C_(6)carbides.In the as-welded state,deformation twins were observed in the weld metal with a fully austenitic structure.The peak micro-hardness was shifted from the ferritic steel side to the weld metal side of the fusion boundary after aging and the peak value increased signific-antly.Based on the experimental results,a mechanism of premature failures of the joints was proposed.

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram(Part 2)

On the base of the methods of predicting weld metal microstructures of pearlitic dissimilar steel welded joints using austenitic type filler materials by Schaeffler Diagram[1], the other new methods of predicting and expressing weld metal microstnictiires of two kinds of dissimilar steel welded joints (pearlite/pearlite and austenite/pearlite) using austenitic filler materials by Schaeffler Diagram are suggested. Those new methods resolve some difficult problems which the microstructure kinds in two heterogeneous mixture zones of weld metal neighbouring two kinds of welded base metals are difficult to be accurately ascertained and the fluctuations of weld metal microstnictiires across fusion line are difficult to be conveniently expressed according to the traditional predicting method. The new predicting methods are more concise and practical.

Investigation of electrode and hydrogen induced crack in welded joint of dissimilar steel

The welded joint of dissimilar heat-resisting steels 20Crl2MoV (F12)and 12 Cr2MoWVTiB(102)generally works around 600°C.In this paper three kinds of ferritic electrodes are used for testing.They are R817 high- strength electrode(CrllMoVNi),R347 low-strength electrode(Cr2MoVWB) and newly-developed R507MoNb medium-strength electrode.The study on the influence of those three different electrodes on carbon migration,HIC and hy- drogen diffusion shows that medium-strength electrodes can well control the carbon migration,and that the tendency to HIC in the joint formed by R817 is smaller than that by R347 instead.Considering the effect of weld metal transfor- mation on the restraint stress and hydrogen concentration of a joint,the hydro- gen distribution in the heat-affected zone(HAZ)is calculated by using finite ele- ment method(FEM)with stress and strain changing,and so the effect of the transformation behaviour on HIC is revealed.In addition,newly-developed R507MoNb electrodes,tested the elevated-temperature property,oxidation re- sistance and creep rupture strength,have fulfilled the technical standards con- cerned and passed the examination of on-the-spot operation.

Microstructure and mechanical properties of dissimilar steel plate resistance plug welding joints

A new type of hybrid welding method called resistance plug welding （RPW） was firstly adopted to achieve the connecting of dissimilar steel, mainly as for the poor welding characteristics of high strength steel produced by increasing carbon, manganese, silicon, etc. Microstructures and mechanical properties of RPW joint were analyzed by optical microscope,micro-hardness test and shear tensile measurement. Experimental results indicate that the RPW joint has a rounded rectangle nugget ^ and the size is larger than elliptical nugget of resistance spot welding （RSW） jo in t; the hardness value of RPW joint is evenly distributed, accordingly there is no hard brittle phases ; the shear tensile strength o f RPW joint increases by 20% in comparison with RSW joint under the same welding conditions.

Effect of welding process on the microstructure and properties of dissimilar weld joints between low alloy steel and duplex stainless steel

To obtain high-quality dissimilar weld joints, the processes of metal inert gas （MIG） welding and tungsten inert gas （TIG） welding for duplex stainless steel （DSS） and low alloy steel were compared in this paper. The microstructure and corrosion morphology of dissimilar weld joints were observed by scanning electron microscopy （SEM）; the chemical compositions in different zones were detected by en- ergy-dispersive spectroscopy （EDS）; the mechanical properties were measured by microhardness test, tensile test, and impact test; the corro- sion behavior was evaluated by polarization curves. Obvious concentration gradients of Ni and Cr exist between the fusion boundary and the type II boundary, where the hardness is much higher. The impact toughness of weld metal by MIG welding is higher than that by TIG weld- ing. The corrosion current density of TIG weld metal is higher than that of MIG weld metal in a 3.5wt% NaC1 solution. Galvanic corrosion happens between low alloy steel and weld metal, revealing the weakness of low alloy steel in industrial service. The quality of joints pro- duced by MIG welding is better than that by TIG welding in mechanical performance and corrosion resistance. MIG welding with the filler metal ER2009 is the suitable welding process for dissimilar metals jointing between UNS $31803 duplex stainless steel and low alloy steel in practical application.

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram

On the base of a number of analyses and researches, some new methods of predicting and expressing the microstructure kinds, of the dissimilar steel welded joint of austenite/pearlite(ferrite) have been presented Those new methods can ascertain the microstructure kind not only in the different characteristic zones of weld metal but also in the different morphologies in the heterogeneous mixture zone of weld metal. Those new methods. enrich and develop the traditional methods of predicting the microstructure of weld metal by Schaeffler Diagram, and are more concise and practical.

Microstructure in the Weld Metal of Austenitic-Pearlitic Dissimilar Steels and Diffusion of Element in the Fusion Zone

Microstructure and alloy element distribution in the welded joint between austenitic stainless steel (1Cr18Ni9Ti) and pearlitic heat-resistant steel (1Cr5Mo) were researched by means of light microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Microstructure, divisions of the fusion zone and elemental diffusion distributions in the welded joints were investigated. Furthermore, solidification microstructure and S-ferrite distribution in the weld metal of these steels are also discussed.

Influence of Creep Strength of Weld on Interfacial Creep Damage of Dissimilar Welded Joint between Martensitic and Bainitic Heat-Resistant Steel

The mechanical properties, creep rupture strength, creep damage and failure characteristics of dissimilar metal welded joint （DMWJ） between martensitic （SA213T91） and bainitic heat-resistant steel （12Cr2MoWVTiB（G102）） have been investigated by means of pulsed argon arc welding, high temperature accelerated simulation, mechanical and creep rupture test, and scanning electronic microscope （SEM）. The results show that there is a marked drop of mechanical properties of undermatching joint, and low ductility cracking along weld/G102 interface is induced due to creep damage. Creep rupture strength of overmatching joint is the least. The mechanical properties of medium matching joint are superior to those of overmatching and undermatching joint, and creep damage and failure tendency along the interface of weld/G102 are lower than those of overmatching and undermatching joint after accelerated simulation for 500 h, 1 000 h, 1 500 h, and the creep rupture strength of medium matching joint is the same as that of undermatching joint. Therefore, it is reasonable that the medium matching material is used for dissimilar welded joint between martensitic and bainitic steel.

Dissimilar Welding of Superduplex Stainless Steel/HSLA Steel for Offshore Applications Joined by GTAW

The dissimilar metal weld is demanding as well as the similar weld, however, dissimilar weld is more complex than similar weld due to the necessity of being applied in zones where a requirement is to improve some properties. In this work the main purpose is to know the mechanical behavior of a dissimilar weld between HSLA Steel and Superduplex Stainless Steel (SDSS) to establish if the joint is feasible or not. The alloys were welded with GTAW process using a 60-deg and 90-deg single-V groove test specimens in order to observe the effect of the weld pass. The filler metal was chosen with the aid of Schaeffler diagram. It was found that the ER 25.10.4L filler metal provided the best equilibrium between ferrite and austenite phase in the Superduplex Stainless Steel final microstructure and a band of martensite in the HSLA steel final microstructure. The dissimilar joint presented acceptable mechanical properties which are superior to the HSLA in the as-received condition, but lower than the SDSS in the as-received condition, proving that the filler metal was the adequate.

Microstructural evolution and mechanical properties of friction stir-welded C71000 copper–nickel alloy and 304 austenitic stainless steel

Dissimilar joints comprised of copper–nickel and steel alloys are a challenge for manufacturers in modern industries, as these metals are not thermomechanically or chemically well matched. The present study investigated the effects of tool rotational speed and linear speed on the microstructure and mechanical properties of friction stir-welded C71000 copper–nickel and 340 stainless steel alloys using a tungsten carbide tool with a cylindrical pin. The results indicated that a rotational-to-linear speed ratio of 12.5 r/mm did not cause any macro defects, whereas some tunneling defects and longitudinal cracks were found at other ratios that were lower and higher. Furthermore, chromium carbide was formed on the grain boundaries of the 304 stainless steel near the shoulder zone and inside the joint zone, directing carbon and chromium penetration toward the grain boundaries. Tensile strength and elongation percentages were 84% and 65% of the corresponding values in the copper–nickel base metal, respectively.

Microstructure characteristics and mechanical properties of steel stud to Al alloy by CMT welding-brazing process

Cold metal transfer （CMT） welding of nickel-coated Q235 steel studs with 606l Al alloy was carried out using ER4043 as filler metal. The welding process was stable, and appearance of weld formed well without surface defect under the parameters of welding current 121 A, welding voltage 15.4 V and welding speed 6 r/min. The microstructure of fiUer metal was analyzed by means of scanning electron microscopy. The filler metal and 6061 Al alloy were fused to form fusion welding interface, the fusion zone had a good bonding without any micro defect. The steel stud did not melt and brazing interface was formed between the filler metal and steel stud. Two different reaction layers existed in the brazing interface, the Fe2Al5 layer about 10 -12 p^m formed near the steel stud side, and the other layer was mainly composed of FeAl3. Nickel-rich zone was formed in the root toe area of the fillet weld, which was mainly composed of Al3Ni2. The tensile tests showed that the maximum shearing strength of the joints was 129 MPa. The joint was brittle fractured in the intermetallic compound layer where plenty of FeAl3 were distributed continuously.

Q345B/304异种钢激光填丝焊接工艺与性能研究

为了提高异种钢焊接质量,采用激光焊技术填充ER308L焊丝对Q345B低合金钢与304不锈钢两种异质材料进行焊接。在不同的激光功率下,采用金相显微镜、扫描电镜、能谱仪等对焊接接头进行了深入的表征和分析。结果表明,当激光功率高于3 kW时,接头处得以焊透,4 kW、4.5 kW和5 kW这3种焊缝截面均呈现束腰现象,且焊缝组织均为典型的板条状马氏体,其中5 kW焊接件焊缝所含的板条马氏体更加细小,表现出最高的硬度值;4 kW焊接件的抗拉强度达到590 MPa,随着激光功率的增大,焊接件的抗拉强度逐步下降;焊缝中主要出现了Cr_(2)Ni_(3)、Fe_(2)Ni_(3)、α-Fe等相,没有大量出现对质量有较大影响的M_(23)C_(6)和σ相等有害相。采用激光填丝焊接技术能够实现Q345B/304异种钢的高质量焊接。

ER50-6焊丝激光焊接Q345B/304异种钢接头组织与性能

目的研究异种钢激光填丝焊接工艺对焊接组织与性能的影响规律,以降低Q345B/304异种钢激光焊接的成本,扩大异种钢激光焊接的应用范围,使其能够满足使用要求,实现Q345B/304异种钢激光焊接低成本的产业化应用。方法采用便宜的ER50-6碳钢焊丝代替价格高昂的ER308L不锈钢焊丝以及不锈钢药芯焊丝对Q345B/304异种钢进行激光焊接,主要采用填丝ER50-6焊丝对5 mm厚的Q345B/304异种钢板进行激光焊接,再结合焊缝区、熔合区及热影响区的纳观-微观-宏观多尺度表征和测试结果,研究不同工艺参数下焊接接头的组织与结构,同时结合焊接工艺参数和多尺度的表征结果,研究焊接接头硬度、焊缝中元素分布、物相特征、力学性能的变化规律。结果当激光功率为4.5 kW、焊接速度为6 mm/s、焊丝直径为1.2 mm、装配间隙为1 mm、送丝速度为2.5 m/min时,焊缝成形最好,且在焊缝中没有出现大量的M_(23)C_(6)和σ等有害相,焊接接头的力学性能等也完全满足使用要求。结论采用ER50-6焊丝对Q345B/304异种钢进行激光焊接,能够得到完全符合质量要求的焊接接头,且降低了焊接成本。

铝/钢异种金属旋转摩擦焊接技术研究进展

铝/钢异种金属结构以其质量轻、设计柔性大的特点在航空航天领域尤其是大推力液体运载火箭输送管路中有着重要应用。相比于熔焊、钎焊等其他焊接方法,旋转摩擦焊可以实现铝/钢异种金属高质、高强、高可靠连接。本文基于对铝/钢异种金属焊接性分析,从工艺参数优化、界面显微组织分析、界面不均匀性调控和添加中间层金属冶金调控等方面综述了铝/钢异种金属旋转摩擦焊接技术研究进展,并对铝/钢异种金属旋转摩擦焊接技术发展趋势及亟须解决的科学问题进行了总结。

钢结硬质合金与钢连接技术的研究现状

钢结硬质合金具有合金钢的优良力学性能和硬质合金的高耐磨性、高硬度的特点,具有良好的性价比,如何将钢结硬质合金与大型耐磨部件金属基体形成牢固的冶金结合,已成为提高设备整体耐磨性和使用寿命的技术关键,通过总结钢结硬质合金与钢的不同冶金结合形式、原理和特点,并概述了近年来国内外学者在此领域所取得的研究成果,具体介绍了镶铸、钎焊、扩散焊、激光焊、电弧焊等连接方法的研究现状,重点总结了这些连接方法的适用范围以及在使用过程中存在的问题,特别是钢结硬质合金与钢基体的界面结合情况进行了统计分析,最后,通过对现有技术的归纳总结,展望了未来实现钢结硬质合金与钢基体良好冶金结合的研究和发展方向.

SUPER304/T92异种钢焊接件热老化的涡流检测

异种钢焊接接头是超(超)临界发电机组管道的薄弱环节,在高温高压作用下易出现力学性能退化甚至失效现象。涡流检测是金属材料检测的一种重要方法,采用涡流法对SUPER304/T92异种钢焊接件的热老化进行检测。首先,研制涡流传感器并搭建了涡流扫查试验平台;其次,开展涡流扫频试验,分析不同激励频率对焊接试件涡流检测能力的影响,优选三种励磁频率下的涡流检测信号,并结合试件的金相显微组织进行分析;最后,研究SUPER304、焊缝和T92三种不同区域的涡流信号随热老化时间的变化关系,并与试件表面硬度进行相关性分析。试验结果为异种钢焊接件的热老化评估提供了一种有效的无损检测方法。

基于粒子群算法确定热源参数的EH40/316L高功率激光焊接温度场模拟

焊接瞬态温度场对焊接残余应力与变形模拟结果具有重要的影响,为提高EH40/316L高功率激光焊接的温度场模拟精度,开发了用于热源模型经验参数优化的智能计算方法.该方法利用MATLAB软件随机生成热源模型的经验参数,调用ANSYS软件执行焊接温度场模拟的APDL命令,并在计算完成后返回结果数据,然后通过借助粒子群算法的群体智能和进化智能的优点,根据预测结果生成新的热源经验参数,进行迭代计算,直至预测结果为最优结果;同时,为提高收敛速度,将有限元几何模型简化为原模型的1/5.基于该方法,开展了10个热源模型经验参数优化案例的计算,并根据优化结果分析了热源模型的参数敏感性.结果表明,该方法实现了温度场的准确预测,其中10个优化案例的平均计算时间为30.3 h,最大、最小和平均预测误差分别为2.84%,2.06%和2.16%;同时,由预测误差和热源经验参数的响应曲面可知,两者之间具有复杂的非线性关系,其数学函数为多谷函数.

坡口形式对Q390/Q690异种低合金钢焊接残余应力的影响

采用有限元法研究了坡口形式(V形、X形和K形)对Q390/Q690异种低合金钢熔化极活性气体保护电弧焊接头温度、应力和焊后残余应力分布的影响,并进行了试验验证。结果表明:模拟和试验所得热循环曲线和残余应力分布相似,瞬时温度相对误差小于15%,证明模型准确;K形坡口接头焊缝区峰值温度最高,V形坡口次之,X形坡口最小;K形、V形和X形坡口接头焊缝均存在等效应力高于690MPa的高应力区,面积依次降低,纵向残余应力均呈“几”字形分布,峰值出现在焊缝处,在两侧熔合区迅速转变为压应力,横向残余应力均呈双驼峰形分布,峰值出现在两侧熔合区,焊缝区应力急剧减小至中心处接近0;坡口形式对接头纵向残余拉应力峰值影响较小,V形与K形坡口接头纵向残余拉应力分布范围大于X形坡口,横向残余拉应力峰值和分布范围均大于X形坡口。

钛与钢异种材料焊接技术的研究进展

钛与钢两种材料在石油化工领域有着广泛的应用。由于它们在物理性能和化学性能方面存在显著差异,焊接时难以获得优异的焊接接头,因此针对钛与钢焊接过程中存在的问题进行分析,从钎焊、熔焊和压力焊三方面对其研究现状进行探讨,并对钛与钢异种材料焊接的发展前景进行展望。